`CKeyStore` is a rich interface that provides many features, including knowledge of scripts and pubkeys for solving, private keys for signing, in addition to watch-only keys and scripts, and distinguishing lack of keys from them just being encrypted.

The signing logic in script/sign does not actually need most of these features. Here we introduce a simpler interface (`SigningProvider`) which *only* provides keys and scripts. This is actually sufficient for signing.

In addtion, we swap the dependency between keystore and script/sign (keystore now depends on script/script with `CKeyStore` deriving from `SigningProvider`, rather than `CKeyStore` being the interface that signing relies on).

This is a very early step towards the design in https://gist.github.com/sipa/125cfa1615946d0c3f3eec2ad7f250a2, separating the concern between deciding what outputs are ours and signing.

make check
test_runner.py